JP2014061274A - Medical image processor and x-ray computerized tomographic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To extract an artifact that occurs in a monochrome X-ray image.SOLUTION: A medical image processor in an embodiment includes a separation part, a reconstruction part, and an extraction part. The separation part separates projection data into respective line integral data of at least two or more reference substances that are previously set. The reconstruction part reconstructs reference substance image data in which the pixel value of each pixel indicates the abundance ratio of a reference substance in the pixel from respective line integral data of the plurality of reference substances. The extraction part extracts an artifact area on the basis of an attenuation coefficient of each pixel calculated from the respective reference substance image data of the plurality of reference substances.

Description

  Embodiments described herein relate generally to a medical image processing apparatus and an X-ray computed tomography apparatus.

  2. Description of the Related Art Conventionally, there is a method of acquiring an image by performing imaging with a plurality of different tube voltages using an X-ray computed tomography (CT) apparatus. When two different tube voltages are used, this technique is called “Dual Energy CT”. In “Dual Energy CT”, two projection data obtained from two different tube voltages are separated into projection data (line integration data) of two preset reference materials, and the two separated data From each, an application technique for reconstructing an image (reference material image) based on the presence rate of the reference material is known. In such applied technology, various images such as a monochromatic X-ray image, a density image, and an effective atomic number image can be acquired by performing weighting calculation processing using two reference substance images.

  The above-described applied technology is effective for correcting artifacts caused by beam hardening. However, as artifacts, there are various artifacts other than beam hardening, such as artifacts caused by deterioration in accuracy of projection data by a high absorber, and artifacts caused by scattered rays.

  In particular, artifacts often occur due to deterioration in the accuracy of projection data by the high absorber. This is because if a substance having a large linear absorption coefficient such as metal exists in the object to be imaged, the count of the detector becomes minute at the time of imaging at a low tube voltage, and correct projection data cannot be obtained. In such a case, the projection data of the reference material cannot be obtained correctly, and as a result, the obtained monochromatic X-ray image has, for example, an artifact that lacks information around the high absorber. With the above applied technology, it has been impossible to generate a monochromatic X-ray image from which the influence of artifacts other than beam hardening has been removed.

JP 2009-261194 A

  The problem to be solved by the present invention is to provide a medical image processing apparatus and an X-ray CT apparatus capable of extracting artifacts generated in a monochromatic X-ray image.

  The medical image processing apparatus according to the embodiment includes a separation unit, a reconstruction unit, and an extraction unit. The separation unit separates the projection data into line integral data of each of at least two or more preset reference materials. The reconstruction unit reconstructs reference material image data in which the pixel value of each pixel indicates the presence rate of the reference material present in the pixel from the line integral data of each of the plurality of reference materials. The extraction unit extracts an artifact region based on the attenuation coefficient of each pixel calculated from the reference material image data of each of the plurality of reference materials.

FIG. 1 is a diagram illustrating an overall configuration example of the X-ray CT apparatus according to the first embodiment. FIG. 2 is a diagram illustrating a configuration example of the preprocessing unit and the image generation unit according to the first embodiment. FIG. 3 is a diagram for explaining the extraction unit according to the first embodiment. FIG. 4 is a diagram illustrating an example of a processing result of the correction unit according to the first embodiment. FIG. 5 is a diagram illustrating an outline of processing according to the first embodiment. FIG. 6 is a flowchart illustrating a processing example of the X-ray CT apparatus according to the first embodiment. FIG. 7A is a diagram (1) for explaining the fourth embodiment. FIG. 7B is a diagram (2) for explaining the fourth embodiment.

  Hereinafter, embodiments of a medical image processing apparatus will be described in detail with reference to the accompanying drawings. In the following, an X-ray computed tomography (CT) apparatus that functions as a medical image processing apparatus will be described as an embodiment.

(First embodiment)
First, an example of the overall configuration of the X-ray CT apparatus according to the first embodiment will be described with reference to FIG. FIG. 1 is a diagram illustrating an overall configuration example of the X-ray CT apparatus according to the first embodiment. As shown in FIG. 1, the X-ray CT apparatus according to the first embodiment includes a gantry device 10, a couch device 20, and a console device 30.

  The gantry device 10 is a device that irradiates a subject P with X-rays and collects X-ray detection data. The gantry device 10 is a high-voltage generator 11, an X-ray tube 12, an X-ray detector 13, and a data collector 14. And a rotating frame 15 and a gantry driving unit 16.

  The high voltage generator 11 is a device that generates a high voltage and supplies the generated high voltage to the X-ray tube 12. The X-ray tube 12 is a vacuum tube that generates X-rays with a high voltage supplied from the high-voltage generator 11, and the X-rays generated by the X-ray tube 12 are irradiated to the subject P.

  The X-ray detector 13 is a detector that detects X-ray detection data indicating the intensity distribution of X-rays irradiated from the X-ray tube 12 and transmitted through the subject P. That is, the X-ray detector 13 detects X-ray detection data indicating the degree of X-ray absorption generated in the subject P. For example, the X-ray detector 13 has a detection element array in which a plurality of X-ray detection elements are arranged along the channel direction (Y-axis direction shown in FIG. 1) in the body axis direction of the subject P (shown in FIG. 1). It is a two-dimensional array type detector arranged in a plurality of rows along the (Z-axis direction).

  The rotating frame 15 supports the X-ray tube 12 and the X-ray detector 13 so as to face each other with the subject P interposed therebetween. The gantry driving unit 16 is a driving device that rotates the rotary frame 15 to rotate the X-ray tube 12 and the X-ray detector 13 on a circular orbit around the subject P.

  The data collection unit 14 is a DAS (Data Acquisition System) and collects X-ray detection data detected by the X-ray detector 13. Specifically, the data collection unit 14 collects X-ray detection data corresponding to each X-ray irradiation direction from the X-ray tube 12. The X-ray irradiation direction is also called a view. Then, the data collection unit 14 performs amplification processing, A / D conversion processing, and the like on the collected X-ray detection data for each view, and outputs the data to a preprocessing unit 34 (described later) of the console device 30. For example, the data collection unit 14 outputs data (sinogram data) in which X-ray detection data indicating the X-ray detection amount for each X-ray detection element is time-series for each X-ray irradiation direction.

  The couch device 20 is a device on which the subject P is placed, and includes a couchtop 22 and a couch driving device 21 as shown in FIG. The top plate 22 is a bed on which the subject P is placed, and the bed driving device 21 rotates the subject P by moving the top plate 22 in the body axis direction (Z-axis direction) of the subject P. Move into frame 15.

  The console device 30 is a device that accepts an operation of the X-ray CT apparatus by an operator and reconstructs a tomographic image from the data group collected by the gantry device 10. As shown in FIG. The display device 32 includes a scan control unit 33, a preprocessing unit 34, a projection data storage unit 35, an image generation unit 36, an image storage unit 37, and a system control unit 38.

  The input device 31 includes a mouse, a keyboard, buttons, a trackball, a joystick, etc. for an operator such as a doctor or engineer who operates the X-ray CT apparatus to input various instructions, and receives various commands received from the operator. Then, the data is transferred to a system control unit 38 to be described later.

  The display device 32 has a monitor for displaying a GUI (Graphical User Interface) for receiving an instruction from an operator via the input device 31 and for displaying an image stored in an image storage unit 37 described later. .

  The scan control unit 33 controls the operations of the high voltage generation unit 11, the gantry driving unit 16, the data collection unit 14, and the bed driving device 21. Thereby, the scan control unit 33 controls the X-ray scan process of the subject P, the X-ray detection data group acquisition process, and the data process for the X-ray detection data group performed by the gantry device 10.

  Specifically, the scan control unit 33 performs X-ray scanning by irradiating the X-ray tube 12 continuously or intermittently while rotating the rotary frame 15. For example, the scan control unit 33 performs helical scanning in which imaging is performed by continuously rotating the rotating frame 15 while moving the top plate 22 or rotating the rotating frame 15 once or continuously while the position of the subject P is fixed. Execute a conventional scan to shoot.

  The preprocessing unit 34 generates projection data by performing correction processing such as logarithmic conversion processing, offset correction, and sensitivity correction on the X-ray detection data transmitted from the data collection unit 14. The processing performed by the preprocessing unit 34 according to the first embodiment will be described in detail later.

  The projection data storage unit 35 stores the projection data generated by the preprocessing unit 34.

  The image generation unit 36 generates various images from the projection data stored in the projection data storage unit 35 and stores the generated images in the image storage unit 37. For example, the image generation unit 36 reconstructs an X-ray CT image by performing a back projection process on the projection data (for example, a back projection process by an FBP (Filtered Back Projection) method), and converts the reconstructed X-ray CT image into an image. Store in the storage unit 37. The processing performed by the image generation unit 36 according to the first embodiment will be described in detail later.

  The system control unit 38 performs overall control of the X-ray CT apparatus by controlling the operations of the gantry device 10, the couch device 20, and the console device 30. Specifically, the system control unit 38 controls the X-ray detection data group collection processing by the gantry device 10 and the couch device 20 by controlling the scan control unit 33. Further, the system control unit 38 controls the image processing in the console device 30 by controlling the preprocessing unit 34 and the image generation unit 36. In addition, the system control unit 38 controls the display device 32 to display various images stored in the image storage unit 37.

  The overall configuration of the X-ray CT apparatus according to the first embodiment has been described above. Under such a configuration, the X-ray CT apparatus according to the first embodiment performs imaging with a tube voltage fixed to one type and collects projection data, and in addition to “Multi Energy” with a plurality of different types of tube voltages. To capture projection data. For example, the X-ray CT apparatus according to the first embodiment collects projection data by performing “imaging with dual energy” at two different tube voltages.

  “Shooting with Dual Energy” is performed, for example, by the following three photographing methods. The first imaging method is a “Slow-kV switching system (two-rotation system)” in which imaging is first performed with a first tube voltage and then with a second tube voltage. The second imaging method uses a dual tube X-ray CT apparatus illustrated in FIG. 1 instead of the one-tube X-ray CT apparatus, and “Dual Source method (two-tube system) that performs imaging at different tube voltages. Method). The third imaging method is a “Fast-kV switching system (high-speed switching system)” that performs imaging by switching the tube voltage at high speed for each view while rotating the rotating frame 15. Thereby, two types of raw data (projection data) having different energies can be acquired.

  Hereinafter, a case where “shooting with Dual Energy” is performed by a high-speed switching method will be described. Note that the present embodiment is applicable even when “shooting with Dual Energy” is performed by the two-rotation method or the two-tube method.

  In recent years, two projection data obtained from two different tube voltages are separated into projection data (line integral data) of two preset reference materials, so that an image based on the presence rate of the reference material (reference Application technology to reconstruct material images has been developed. In such applied technology, various images such as a monochromatic X-ray image, a density image, and an effective atomic number image can be acquired by performing weighting calculation processing using two reference substance images.

  The above-mentioned applied technology is effective in correcting artifacts caused by beam hardening. For example, X-rays composed of monochromatic X-rays that are less affected by beam hardening than conventional X-ray CT images composed of continuous X-rays. A line CT image (monochromatic X-ray image or monochromatic X-ray CT image) can be generated. However, as artifacts, there are various artifacts other than beam hardening, such as artifacts caused by deterioration in accuracy of projection data by a high absorber, and artifacts caused by scattered rays.

  With the above applied technology, it has been impossible to generate a monochromatic X-ray image from which the influence of artifacts other than beam hardening has been removed. Therefore, in the first embodiment, processing of the preprocessing unit 34 and the image generation unit 36 described below is performed in order to extract artifacts generated in the monochromatic X-ray image.

  FIG. 2 is a diagram illustrating a configuration example of the preprocessing unit and the image generation unit according to the first embodiment. As illustrated in FIG. 2, the preprocessing unit 34 according to the first embodiment includes a projection data generation unit 34a and a separation unit 34b. Further, as illustrated in FIG. 2, the image generation unit 36 according to the first embodiment includes a reconstruction unit 36a, an extraction unit 36b, and a correction unit 36c.

  The projection data generation unit 34a performs a logarithmic conversion process or the like on the X-ray detection data transmitted from the data collection unit 14 to generate projection data. In the first embodiment, the projection data generation unit 34a generates projection data (hereinafter referred to as high energy projection data) from X-ray detection data having a first tube voltage (for example, 130 kV). In the first embodiment, the projection data generation unit 34a generates projection data (hereinafter referred to as low energy projection data) from the X-ray detection data of the second tube voltage (for example, 80 kV).

  The separation unit 34b separates the projection data into line integral data of each of at least two or more preset reference materials. In the first embodiment, the projection data is two pieces of projection data (high energy projection data and low energy projection data) collected respectively from two different types of tube voltages. In the first embodiment, the plurality of reference substances are two reference substances, for example, bone and water. Hereinafter, one of the two reference substances is described as a first reference substance, and the other is described as a second reference substance.

  That is, the separation unit 34b converts the high energy projection data and the low energy projection data into the line integration data (first line integration data) of the first reference material and the line integration data (second line integration data) of the second reference material. And to separate. The reference substance is set from substances whose mass attenuation coefficients at various energies are known.

  The first line integration data and the second line integration data separated by the separation unit 34 b are stored in the projection data storage unit 35.

Then, the reconstruction unit 36a reconstructs the reference material image data in which the pixel value of each pixel (pixel or voxel) indicates the presence rate of the reference material existing in the pixel from the line integral data of each of the plurality of reference materials. . Specifically, the reconstruction unit 36a reconstructs the reference material image data of the first reference material (hereinafter referred to as the first reference material image data) by performing a back projection process on the first line integration data. In addition, the reconstruction unit 36a reconstructs the reference material image data (hereinafter referred to as second reference material image data) of the second reference material by performing back projection processing on the second line integration data. Here, the pixel value of the pixel “i” in the first reference substance image data is the abundance ratio “c ₁ ” of the first reference substance in the pixel “i”. Further, the pixel value of the pixel “i” of the second reference material image data is the abundance ratio “c ₂ ” of the second reference material in the pixel “i”.

Here, the attenuation coefficient “μ (E)” of the imaging region corresponding to the pixel “i” at an arbitrary energy “E” can be obtained by the following equation (1). In the following formula (1), “μ ₁ (E)” is an attenuation coefficient in “E” of the first reference substance, and “μ ₂ (E)” is “E” of the second reference substance. Is the attenuation coefficient.

The CT value “CT # (E)” of the imaging region corresponding to the pixel “i” in “E” is “μ (E)” calculated by the equation (1) and the attenuation coefficient in “E” of water. It is obtained by substituting “μ _w (E)” into the following equation (2).

  The reconstruction unit 36a can generate a monochromatic X-ray image at an arbitrary energy “E” using the reference material image data and the equations (1) and (2). In the attenuation coefficient obtained by the equation (1), an error due to beam hardening is reduced. However, the attenuation coefficient obtained by the equation (1) remains affected by, for example, metal artifacts, artifacts caused by bones and contrast agents, and cone beam artifacts.

  2 extracts the artifact region based on the attenuation coefficient of each pixel calculated from the reference material image data of each of the plurality of reference materials. The extraction unit 36b according to the first embodiment extracts an artifact region by comparing attenuation coefficients at two different energies in an energy range that does not include the absorption edge energy. FIG. 3 is a diagram for explaining the extraction unit according to the first embodiment.

The extraction unit 36b according to the first embodiment calculates the attenuation coefficient of each of the _two energies (E ₁ and E ₂ ) for each pixel using Expression (1). The magnitude relationship between the two energies is “E ₁ <E ₂ ”. Here, the mass attenuation coefficient (linear attenuation coefficient / density) of the substance has the shape shown in FIG. 3 with respect to the photon energy. In FIG. 3, the mass attenuation coefficient of water is indicated by a solid line, the mass attenuation coefficient of bone (cortical bone) is indicated by a dotted line, and the mass attenuation coefficient of iodine is indicated by an alternate long and short dash line.

As shown in FIG. 3, in the energy region where the photoelectric effect or Compton scattering is dominant, any substance except “near absorption edge energy where the attenuation coefficient becomes discontinuous” becomes “μ (E ₁ )> μ (E ₂ ) ”Is established. Using this fact, the extraction unit 36b determines that correct “c ₁ ” and “c ₂ ” are not obtained in pixels where “μ (E ₁ )> μ (E ₂ )” is not satisfied, The pixel is extracted as an artifact area. In other words, when the magnitude relationship between the attenuation coefficients of the two energies calculated from the “c ₁ ” and “c ₂ ” of a certain pixel is a physically inconsistent magnitude relationship, the extraction unit 36 b It is determined that the area is an artifact area.

  The two energies are set by the operator within a range excluding the absorption edge energies of the set two reference substances. Alternatively, the two energies may be preset in the apparatus in advance according to the reference material pair. Alternatively, the two energies may be set by the extraction unit 36b in accordance with a reference material pair.

2 corrects the attenuation coefficient of the artifact region. For example, the correction unit 36c corrects the attenuation coefficient of the artifact region by using the points described below. That is, “c ₁ ” and “c ₂ ” of the artifact region are not correct, but among the attenuation coefficients at various energies calculated from the incorrect “c ₁ ” and “c ₂ ”, certain energy “E_cor” The attenuation coefficient obtained in (1) is a reasonable value.

  In the range excluding the absorption edge energy, the correct attenuation coefficient value decreases as the energy increases. That is, the correct attenuation coefficient graph has a downward-sloping shape. On the other hand, the attenuation coefficient of the artifact region extracted by the first embodiment increases as the energy increases even in the range excluding the absorption edge energy. That is, the attenuation coefficient graph of the artifact region has a shape that rises to the right. The graph of the correct attenuation coefficient having a right-down shape and the graph of the attenuation coefficient of the artifact region having a right-up shape intersect at a certain point. The energy at the intersection is the above “E_cor”.

  Therefore, the correction unit 36c acquires energy “E_cor” in which the attenuation coefficient of the artifact region calculated from the pixel values of the reference substance image data of each of the plurality of reference substances is substantially correct. In the present embodiment, the correction unit 36c acquires energy “E_cor” in which the attenuation coefficient of the artifact region calculated from the pixel values of the first reference material image data and the second reference material image data is substantially correct. . Then, the correction unit 36c performs correction processing using the attenuation coefficient of the artifact region at “E_cor” and the attenuation coefficient of the predetermined substance at “E_cor”.

  For example, the correction unit 36c acquires an energy value obtained in advance experimentally or experimentally as “E_cor”. In such a case, for example, the value of “E_cor” is initially set in the system control unit 38, and the correction unit 36 c acquires “E_cor” from the system control unit 38. Alternatively, the value of “E_cor” may be set by the operator via the input device 31 when the correction unit 36c is processed. In such a case, the correction unit 36 c acquires “E_cor” set using the input device 31 via the system control unit 38.

However, the energy “E_cor” that provides a reasonable attenuation coefficient is not necessarily the same value. For this reason, in this embodiment, the correlation between the attenuation coefficient “μ (A)” and “E_cor” of a certain energy “A” is obtained in advance. Note that “A” is energy set in advance in accordance with the reference substance pair in a range excluding the absorption edge energy. In addition, “μ (A)” represents “c ₁ ” and “c ₂ ”, the attenuation coefficient of the first reference substance in “A”, and the attenuation coefficient of the second reference substance in “A” (1 ) Is obtained by substituting for.

  As an example, in the first embodiment, using the slope “a” and the Y intercept “b”, “E_cor” and “μ (A)” are linear functions as in the following equation (3). The fact that the relational expression that can be expressed holds is used.

  “A”, “a”, and “b” are experimentally obtained values. In the present embodiment, the relational expression between “E_cor” and “μ (A)” is described as a linear function. However, the relational expression between “E_cor” and “μ (A)” is described. May be represented by various functions such as a polynomial function, an exponential function, and a logarithmic function. As described above, when the correction process is performed with the value of “E_cor” set to a constant value, Equation (3) is set to “a = 0, b = E_cor”.

The correction unit 36c calculates “E_cor” by substituting “μ (A)” calculated from “c ₁ ” and “c ₂ ” of the pixel extracted as the artifact region into Expression (3). Then, the correction unit 36c substitutes the attenuation coefficient of the first reference substance and the attenuation coefficient of the second reference substance, and “c ₁ ” and “c ₂ ” in “E_cor” into the equation (1), ”Is calculated.“ Μ (E_cor) ”is calculated. The attenuation coefficient “μ (E_cor)” is an attenuation coefficient of the artifact region at “E_cor”, and can be used as a value close to the true attenuation coefficient of the artifact region at “E_cor”.

Then, the correction unit 36c performs correction processing using the attenuation coefficient “μ (E_cor)” and the attenuation coefficient of the predetermined substance at “E_cor”. For example, the predetermined substance is set to water. In such a case, the correction unit 36c includes a water attenuation coefficient “μ _w (E)” for energy “E” corresponding to the monochromatic X-ray image, a water attenuation coefficient “μ _w (E_cor)” for “E_cor”, “Μ (E_cor)” is substituted into the following equation (4). Thereby, the correction unit 36c obtains the attenuation coefficient “μ ′ (E)” after correction of the pixel extracted as the artifact region.

  In the above formula (4), the attenuation coefficient of water is used, but in the present embodiment, the corrected attenuation coefficient may be calculated using the attenuation coefficient of other appropriate substances. Here, also in the case where correction processing is performed with “E_cor” set to a constant value, the attenuation coefficient after correction of the artifact region is obtained using the above equation (4).

  FIG. 4 is a diagram illustrating an example of a processing result of the correction unit according to the first embodiment. In FIG. 4, the graph of the attenuation coefficient (line attenuation coefficient) before correction of the artifact region is indicated by a solid line, and the graph of the attenuation coefficient (line attenuation coefficient) after correction of the artifact region is indicated by a dotted line. By performing the correction process of the correction unit 36c, the shape of the attenuation coefficient graph before the correction, which has risen to the right, becomes the shape of the lower right, which is not physically contradictory, as illustrated in FIG.

  The reconstruction unit 36a shown in FIG. 2 generates a monochrome X-ray image using the corrected attenuation coefficient. Specifically, in the artifact region, the reconstruction unit 36a calculates the CT value by substituting the corrected attenuation coefficient into Equation (2). Further, the reconstruction unit 36a calculates the attenuation coefficient using the formula (1) from the pixel values of the first reference material image data and the second reference material image data in the region other than the artifact region, and calculates the calculated attenuation. The CT value is calculated by substituting the coefficient into equation (2). Thereby, the reconstruction unit 36a generates a monochrome X-ray image of energy “E”.

  Under the control of the system control unit 38, the display device 32 displays a monochrome X-ray image of energy “E”.

  FIG. 5 is a diagram illustrating an outline of processing according to the first embodiment. As shown in the upper diagram of FIG. 5, when the correction process of the correction unit 36 c is not performed, the monochromatic X-ray image has whitening artifacts and blackening artifacts due to deterioration in accuracy of projection data by the high absorber. .

  As shown in the middle diagram of FIG. 5, all such areas are extracted as artifact areas by the extraction process of the extraction unit 36b. Then, by performing the correction process of the correction unit 36c, a monochromatic X-ray image in which the artifact region due to the high absorber is corrected is generated as shown in the lower diagram of FIG.

  When the correction process is performed with “E_cor” set to a constant value, the operator may change the value of “E_cor”. For example, the operator refers to a monochrome X-ray image generated using the attenuation coefficient after correction with the energy “E” of the artifact region, and the operator does not properly correct the monochrome X-ray image. If it is determined, the value of “E_cor” is changed. In such a case, the correction unit 36c performs attenuation coefficient correction processing again using “E_cor” after the change, and the reconstruction unit 36a generates a monochrome X-ray image using the attenuation coefficient after recorrection. When correction processing is performed with “E_cor” as a constant value, a plurality of values are set as “E_cor”, and correction processing is performed for each of these values to generate a plurality of monochrome X-ray images. good. In such a case, for example, the operator can select a monochromatic X-ray image in which artifacts are appropriately reduced from a plurality of monochromatic X-ray images. In addition, even when “E_cor” is acquired using Expression (3) and the attenuation coefficient of the artifact region is corrected using “E_cor” acquired using Expression (3) and Expression (4), the operator May change the value of “E_cor”.

  In the present embodiment, the correction process may be performed by the following process that does not use Expression (3) and Expression (4). For example, the correction unit 36c may perform a correction process for replacing the attenuation coefficient of the artifact region with the attenuation coefficient of a predetermined substance. In such a case, the correction unit 36c performs the correction process by replacing the attenuation coefficient of the artifact region with the attenuation coefficient of an arbitrary substance. As an arbitrary substance, a soft tissue is mentioned, for example.

  Alternatively, the correction unit 36c corrects the attenuation coefficient of the artifact region by correcting the projection data and the line integration data that pass through the artifact region. For example, the correction unit 36c corrects the high energy projection data and the low energy projection data passing through the artifact region, and again separates the corrected projection data into the first line integration data and the second line integration data in the separation unit 34b. Let Alternatively, for example, the correction unit 36c corrects the first line integration data and the second line integration data that pass through the artifact region. Then, the correction unit 36c again causes the reconstruction unit 36a to reconstruct the reference material image data from the first line integration data and the second line integration data. Thereby, the correction | amendment part 36c can obtain | require the attenuation coefficient after correction | amendment.

  Next, an example of processing of the X-ray CT apparatus according to the first embodiment will be described with reference to FIG. FIG. 6 is a flowchart illustrating a processing example of the X-ray CT apparatus according to the first embodiment.

  As illustrated in FIG. 6, the separation unit 34 b of the X-ray CT apparatus according to the first embodiment converts the collected high energy projection data and low energy projection data into first line integration data and second line integration data. Separate (step S101). Then, the reconstruction unit 36a reconstructs the first reference material image data and the second reference material image data from the first line integration data and the second line integration data, respectively (step S102).

  Then, the extraction unit 36b extracts an artifact region from the attenuation coefficient of each pixel calculated from the first reference material image data and the second reference material image data (step S103), and the correction unit 36c attenuates the artifact region. The coefficient is corrected (step S104).

  Then, the reconstruction unit 36a generates a monochrome X-ray image using the corrected attenuation coefficient (step S105). Then, the display device 32 displays a monochromatic X-ray image (step S106) and ends the process.

  As described above, in the first embodiment, pixels whose attenuation coefficient values calculated from the reference substance image data are physically contradictory values are extracted as artifact regions. Thereby, in 1st Embodiment, the artifact which generate | occur | produced in the monochrome X-ray image can be extracted. In the first embodiment, the attenuation coefficient of the artifact region is corrected, and a monochromatic X-ray image is generated using the corrected attenuation coefficient. That is, in the first embodiment, it is possible to correct the weighting coefficient when generating a monochromatic X-ray image. Thereby, in 1st Embodiment, the artifact in a monochromatic X-ray image can be reduced.

(Second Embodiment)
In the second embodiment, another embodiment of the artifact region extraction method performed by the extraction unit 36b will be described. Note that the artifact region extraction method described in the first embodiment is the first extraction method, and the artifact region extraction method performed by the extraction unit 36b according to the second embodiment is largely the second to sixth extraction methods. It explains separately.

The second extraction method is a method that utilizes the fact that it is physically impossible that the attenuation coefficient is 0 or less. In the second extraction method, the extraction unit 36b extracts pixels whose attenuation coefficient can be 0 or less as an artifact region. The energy range used for extraction is set from the set first reference material and second reference material and the range of tube voltage that can be irradiated by the X-ray CT apparatus. In the following description, it is assumed that the energy range is “E _{a to} E _b ”.

For example, the extraction unit 36b sequentially calculates attenuation coefficients in the range of “E _{a to} E _b ” from “c ₁ ” and “c ₂ ” of each pixel and Expression (1). Then, the extraction unit 36b extracts pixels having a combination of “c ₁ ” and “c ₂ ” for which attenuation coefficients of 0 or less are calculated in the range of “E _{a to} E _b ” as artifact regions. However, in the above method, it is necessary to calculate all attenuation coefficients in the range of “E _{a to} E _b ”. Therefore, in the second extraction method, in order to reduce the load of extraction processing, when a combination of “c ₁ ” and “c ₂ ” of a certain pixel is the following three, the pixel is extracted as an artifact region. First, the first case is a case where both “c ₁ ” and “c ₂ ” are 0 or less.

In the second case, “c ₁ ” Is a negative value and the maximum value of “μ ₂ (E) / μ ₁ (E)” in the range of “E _{a to} E _b ” is “R”, the following equation (5) is obtained. This is the case.

In the third case, “c ₂ ” Is a negative value and the maximum value of “μ ₁ (E) / μ ₂ (E)” in the range of “E _{a to} E _b ” is “R ′”, the following equation (6) Is true.

Since the values of “c ₁ ” and “c ₂ ” are known for all pixels, and “R” and “R ′” are also known, determination using the first to third cases By performing the above, in the second extraction method, the load of the extraction process can be reduced.

  Next, the third extraction method and the fourth extraction method will be described. In the third extraction method and the fourth extraction method, the extraction unit 36b extracts an artifact region by comparing an attenuation coefficient at a predetermined energy with an attenuation coefficient at a predetermined energy of the substance. Here, the predetermined energy is “E ′”. “E ′” is a value set by the operator or a default value.

In the third extraction method, the substance that maximizes the X-ray absorption that can exist inside the human body (maximum absorption substance) is set as the preset substance. In the third extraction method, the extraction unit 36b determines that the attenuation coefficient “μ (E ′)” in “E ′” calculated by the equation (1) is larger than the attenuation coefficient in “E ′” of the maximum absorption material “ Pixels having a combination of “c ₁ ” and “c ₂ ” are extracted as artifact regions.

In the fourth extraction method, a substance that minimizes X-ray absorption (minimum absorption substance) that can exist in the human body is set as the above-described preset substance. In the fourth extraction method, the extraction unit 36b determines that the attenuation coefficient “μ (E ′)” in “E ′” calculated by Expression (1) is smaller than the attenuation coefficient in “E ′” of the minimum absorption material. Pixels having a combination of “c ₁ ” and “c ₂ ” are extracted as artifact regions.

Next, the fifth extraction method and the sixth extraction method will be described. In the fifth extraction method and the sixth extraction method, the extraction unit 36b compares the ratio of the attenuation coefficient in each of the two different energies with the ratio of the attenuation coefficient in each of the two different energies of the preset substance. Extract artifact regions. Here, it is assumed that two different energies are “E ₃ and E ₄ , where E ₃ <E ₄ ”. “E ₃ and E ₄ ” are values set by the operator or initially set values.

In the fifth extraction method, the maximum absorption substance is the aforementioned preset substance. In the fifth extraction method, extraction unit 36b, the formula (1) attenuation coefficient "mu (E _3)" in the "E _3" as calculated by the the "E _4" attenuation coefficient in "mu (E _4)" The ratio “μ (E ₃ ) / μ (E ₄ )” is calculated. The extraction unit 36b, attenuation coefficient "mu _a (E _3)" in the "E _3" at the maximum absorption material and attenuation coefficient "μ _a (E _4)" in the "E _4" and the ratio "mu _a (E ₃ ) / μ _a (E ₄ ) ”is acquired. The combination of extraction unit 36b, _{"μ (E 3) / μ (} E 4) " is _{"μ a (E 3) / μ} a (E 4) " becomes greater than "c _1" and "c _2" Are extracted as artifact regions.

In the sixth extraction method, the minimum absorption substance is the preset substance. In the sixth extraction method, extraction unit 36b, the formula (1) attenuation coefficient "mu (E _3)" in the "E _3" as calculated by the the "E _4" attenuation coefficient in "mu (E _4)" The ratio “μ (E ₃ ) / μ (E ₄ )” is calculated. The extraction unit 36b, attenuation coefficient "mu _b (E _3)" in the "E _3" minimum absorbing material with attenuation coefficient "μ _b (E _4)" in the "E _4" and the ratio "mu _b (E ₃ ) / μ _b (E ₄ ) ”. The combination of extraction unit 36b, _{"μ (E 3) / μ (} E 4) " is _{"μ b (E 3) / μ} b (E 4) " smaller than "c _1" and "c _2" Are extracted as artifact regions.

  The first extraction method to the sixth extraction method may be performed independently or in a combination of two or more. By performing a combination of the first extraction method to the sixth extraction method, the extraction accuracy of the artifact region can be improved. Even after the artifact region is extracted by the method described in the second embodiment, the correction process and the monochrome X-ray image generation process described in the first embodiment are performed.

(Third embodiment)
In the third embodiment, a method for further improving the artifact region extraction accuracy performed by the extraction unit 36b will be described.

When the extraction unit 36b according to the third embodiment performs the first extraction method to the sixth extraction method, as the seventh extraction method, the pixel of the pixel at the same position in the reference material image data of each of the plurality of reference materials Pixels whose values fall within a predetermined range are excluded from the artifact region extraction targets. For example, the extraction unit 36b excludes pixels in which “c ₁ ” and “c ₂ ” are both in the “0 ± α” range. Note that “α” is a value set by the operator or a default value.

  By performing the seventh extraction method, in the third embodiment, it can be avoided that the air region is extracted as noise.

  Furthermore, when performing the first extraction method to the sixth extraction method, the extraction unit 36b according to the third embodiment performs the following with the seventh extraction method or without executing the seventh extraction method. An eighth extraction method may be performed.

  In the eighth extraction method, the extraction unit 36b according to the third embodiment extracts an artifact region from the data after the filter processing is performed on the reference material image data of each of the plurality of reference materials. For example, the extraction unit 36b extracts an artifact region after performing a filtering process such as a median filter on the reference material image data. The filtering process may be executed by a processing unit other than the extraction unit 36b.

  By performing the eighth extraction method, isolated points due to noise in the reference material image data can be eliminated in the third embodiment.

(Fourth embodiment)
In the fourth embodiment, a case where the artifact region extracted by the extraction unit 36b is clearly shown to the observer will be described with reference to FIGS. 7A and 7B. 7A and 7B are diagrams for explaining the fourth embodiment.

  In the fourth embodiment, the system control unit 38 performs control so that the artifact region is highlighted in the monochromatic X-ray image using the corrected attenuation coefficient. For example, under the control of the system control unit 38, the reconstruction unit 36a draws the outline of the artifact region in the monochromatic X-ray image using the corrected attenuation coefficient with a dotted line as shown in FIG. 7A. The display device 32 displays the image shown in FIG. 7A under the control of the system control unit 38.

  Alternatively, in the fourth embodiment, the system control unit 38 performs control so that the artifact region is highlighted in the monochromatic X-ray image using the attenuation coefficient before correction. For example, under the control of the system control unit 38, the reconstruction unit 36a draws the outline of the artifact region in the monochromatic X-ray image using the attenuation coefficient before correction with a dotted line as shown in FIG. 7B. The display device 32 displays the image shown in FIG. 7B under the control of the system control unit 38.

  Note that the system control unit 38 may control to display a monochromatic X-ray image in which the extracted artifact region is colored.

  In the fourth embodiment, when the artifact region is highlighted in the monochromatic X-ray image using the attenuation coefficient before correction, the correction coefficient 36c may not be corrected. .

  In the fourth embodiment, since an artifact area in a monochromatic X-ray image is visualized, an area in which information may be missing due to a high-absorber, for example, to an interpreting doctor who interprets a monochromatic X-ray image Can be presented.

  Note that the medical image processing methods described in the first to fourth embodiments can be applied even when performing “multi energy imaging” with three or more different tube voltages. The medical image processing methods described in the first to fourth embodiments can be applied even when three or more reference substances are set.

  In the first to fourth embodiments, the case where the X-ray detector 13 is an integral detector has been described. However, in the medical image processing methods described in the first to fourth embodiments, the X-ray detector 13 detects a photon counting method in which light derived from X-rays transmitted through the subject P is individually counted. Even if it is a container, it is applicable. When the X-ray detector 13 is a photon counting type detector, the separation unit 34b can obtain a line attenuation coefficient from projection data collected by performing imaging with the tube voltage fixed to one type.

  In addition, the medical image processing methods described in the first to fourth embodiments may be executed in a medical image processing apparatus installed separately from the X-ray CT apparatus. In such a case, the medical image processing apparatus receives the projection data collected by the X-ray CT apparatus and executes the above-described medical image processing method.

  Further, each component of each illustrated apparatus is functionally conceptual, and does not necessarily need to be physically configured as illustrated. In other words, the specific form of distribution / integration of each device is not limited to that shown in the figure, and all or a part thereof may be functionally or physically distributed or arbitrarily distributed in arbitrary units according to various loads or usage conditions. Can be integrated and configured. Further, all or a part of each processing function performed in each device may be realized by a CPU and a program analyzed and executed by the CPU, or may be realized as hardware by wired logic.

  As described above, according to the first to fourth embodiments, artifacts generated in a monochromatic X-ray image can be extracted.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 10 Stand apparatus 11 High voltage generation part 12 X-ray tube 13 X-ray detector 14 Data collection part 15 Rotating frame 16 Mount drive part 20 Bed apparatus 21 Bed drive apparatus 22 Top plate 30 Console apparatus 31 Input apparatus 32 Display apparatus 33 Scan control Unit 34 preprocessing unit 34a projection data generation unit 34b separation unit 35 projection data storage unit 36 image generation unit 36a reconstruction unit 36b extraction unit 36c correction unit 37 image storage unit 38 system control unit

Claims (14)

A separation unit that separates projection data into line integral data of each of a plurality of reference materials set in advance at least two or more;
A reconstructing unit that reconstructs reference material image data in which the pixel value of each pixel indicates the abundance of the reference material present in the pixel from the line integral data of each of the plurality of reference materials;
Based on the attenuation coefficient of each pixel calculated from the reference material image data of each of the plurality of reference materials, an extraction unit that extracts an artifact region;
A medical image processing apparatus comprising: A correction unit for correcting the attenuation coefficient of the artifact region;
Further comprising
The medical image processing apparatus according to claim 1, wherein the reconstruction unit generates a monochrome X-ray image using the corrected attenuation coefficient.   The correction unit obtains energy at which the attenuation coefficient of the artifact region calculated from the pixel values of the reference material image data of each of the plurality of reference materials becomes a substantially correct attenuation coefficient, and attenuates the artifact region with the energy The medical image processing apparatus according to claim 2, wherein correction processing is performed using a coefficient and an attenuation coefficient of a predetermined substance at the energy.   The correction unit acquires a value obtained in advance as energy at which an attenuation coefficient of the artifact region calculated from a pixel value of reference substance image data of each of the plurality of reference substances becomes a substantially correct attenuation coefficient. The medical image processing apparatus according to 3.   The medical image processing apparatus according to claim 2, wherein the correction unit performs correction processing for replacing an attenuation coefficient of the artifact region with an attenuation coefficient of a predetermined substance.   The said extraction part extracts the said artifact area | region from the data after the filter process was performed with respect to the reference substance image data of each of these reference substances, The statement of any one of Claims 1-5 Medical image processing apparatus.   The medical image according to claim 1, wherein the extraction unit extracts the artifact region by comparing attenuation coefficients at two different energies in an energy range that does not include absorption edge energy. Processing equipment.   The medical image processing apparatus according to claim 1, wherein the extraction unit extracts a pixel whose attenuation coefficient can be 0 or less as the artifact region.   The said extraction part extracts the said artifact area | region by comparing the attenuation coefficient in the predetermined | prescribed energy and the attenuation coefficient in the said energy of the preset substance. Medical image processing apparatus.   The said extraction part extracts the said artifact area | region by comparing the ratio of the attenuation coefficient in each of two different energy, and the ratio of the attenuation coefficient in each of the said two different energy of the preset substance. The medical image processing apparatus as described in any one of -6.   The reference unit according to claim 1, wherein in the reference material image data of each of the plurality of reference materials, the extraction unit excludes a pixel in which a pixel value of a pixel at the same position falls within a predetermined range from an extraction target of the artifact region. The medical image processing apparatus according to any one of the above.   The medical image processing apparatus according to any one of claims 1 to 11, wherein the projection data is two pieces of projection data each collected by two different types of tube voltages. A control unit for controlling the artifact region to be highlighted in a monochrome X-ray image using the corrected attenuation coefficient or a monochrome X-ray image using the attenuation coefficient before correction;
The medical image processing apparatus according to any one of claims 2 to 12, further comprising: A separation unit that separates projection data into line integral data of each of a plurality of reference materials set in advance at least two or more;
A reconstructing unit that reconstructs reference material image data in which the pixel value of each pixel indicates the abundance of the reference material present in the pixel from the line integral data of each of the plurality of reference materials;
Based on the attenuation coefficient of each pixel calculated from the reference material image data of each of the plurality of reference materials, an extraction unit that extracts an artifact region;
An X-ray computed tomography apparatus comprising:

Images